Vrrrr ... faster than light ... or wind?

Vrrrr ... faster than light ... or wind?

Vrrrr ... or the wind?

I personally love slippers, bicycles, roller skates and long walks but we don't all think the same way. In fact, the diabolical human mind thinks one and makes a hundred of them. The supersonic planes weren't enough, now we are the supersonic boats ... In fact the peaceful English (it seems incredible right?) Have invented a sailboat in the shape of a tapered cross that mounts a sail on a seven meters high mast that manages to go beyond the 300 km / h! Surely the current 150km / h on land and 230km / h on ice looked downright ridiculous!

Vrrrr ... faster than light ... or wind?

. perhaps the boat is moving with the apparent wind.

The story that in order to make an object go at a certain speed, one must push it with another object that has at least the same speed is simply false. Just think of Newton's famous apple falling from the tree. Is there a 'hand' that moves at a certain speed that pushes it? It is only a question of 'external forces' and not 'external speeds': an external force is enough and an object accelerates until there is another force external to the first that opposes it.
The wind, if it hits them well, exerts a force on the sails and makes them accelerate together with the boat to which they are attached.In theory the speed of the boat could increase indefinitely but other factors intervene that modify or contrast this force (apparent wind, change of angle of incidence and friction force) for which the acceleration at a certain point is canceled and the boat, when the sum of all the forces is zero, continues to proceed with uniform motion (maybe.).
The relationship between this limiting speed and the wind speed depends on non-obvious factors and, a priori, it is not possible to state that the limiting speed of a sail must necessarily be less than the wind speed.

After that the spinnaker is another thing, in the sense that it is not a sail in the sense given above.

Vrrrr ... faster than light ... or wind?

Thunderstorms are not usually catastrophic events - about 100,000 occur annually in the United States and only 10 percent of them are severe. The average wind speed during a thunderstorm varies and depends on the temperature, humidity, topography and phase of the storm itself. The speed is highest when the storm produces more rain and lightning. A storm gains a severe rating when its wind speed exceeds 58 miles per hour.

TL DR (Too long, not read)

About 10,000 thunderstorms in the United States are classified as severe each year. This happens when wind speeds exceed 58 miles per hour. The average wind speed during a thunderstorm varies and depends on the temperature, humidity, topography and phase of the storm itself.

Two wind movements occur during a storm: an updraft of warm air, which predominates during the formation and maturation of the storm, and a downdraft of colder air that becomes more prominent as the storm dissipates. The strongest winds occur during the midpoint of the storm, when these opposites are approximately equal.

The modern version of the Beaufort scale includes 12 designations, each of which corresponds to a range of wind speeds. Designations 6 to 10 represent typical wind conditions during an average thunderstorm - 22 to 55 miles per hour.

A thunderstorm requires warm, humid air and a mass of colder air that can push it upwards. As the warm air rises, the contained humidity cools, condenses and falls to the earth in the form of rain. Meanwhile, the friction of air molecules rushing each other creates an electrical charge that eventually discharges like lightning. Two wind movements occur during a storm: an updraft of warm air, which predominates during the formation and maturation of the storm, and a downdraft of colder air that becomes more prominent as the storm dissipates. The strongest winds occur during the midpoint of the storm, when these opposites are approximately equal.

In 1806, Commander Francis Beaufort of the British Navy transcribed his version of a wind scale that was already in widespread use, and meteorologists have since used the Beaufort scale to measure wind speed. The modern version of the scale includes 12 designations, each of which corresponds to a range of wind speeds. The first two indicate speeds typical of severe thunderstorms and hurricanes, while the other ten represent the ascending speed from calm winds to gale force winds. In particular, the designations 6 to 10 on the scale represent typical wind conditions during an average storm. Speeds represented range from 35 to 88 kilometers per hour (22 to 55 miles per hour).

The National Oceanographic and Atmospheric Administration classifies thunderstorms as severe when accompanied by hailstones larger than three-quarters of an inch in diameter and tornadoes or wind speeds greater than 93 kilometers per hour (58 miles per hour) . Most storms, however, don't have as strong winds. In fact, the winds in most storms never go beyond the intensity indicated by an 8 on the Beaufort scale, which is fast enough to break twigs from trees and make walking against the wind very difficult. Wind speeds represented by an 8 are in the range of 54 to 64 kilometers per hour (39 to 46 miles per hour).

Average speed during a storm

A storm ranging from dead calm to a wind intensity measuring 8 on the Beaufort scale would, on average, have a wind speed of about 32 kilometers per hour (20 miles per hour). The average speed of a severe thunderstorm that begins with dead calm, on the other hand, could have an average speed of 50 kilometers per hour (31 miles per hour). During the later stages, some storms show severe rollovers with winds that can exceed 161 kilometers per hour (100 miles per hour). These dangerous downdrafts, which exceed the maximum wind speed of most storms and are as fast as tornadoes, pose a danger to aircraft.

Vrrrr ... faster than light ... or wind?

The most important Sun-Earth relationship is certainly that according to which our very existence is ensured by the continuous and constant flow of energy, which provides livable temperatures, although with differences from place to place and from season to season. However, other phenomena affect the Earth, mainly due to the solar wind and solar activity.
The swarms of solar wind particles, interacting with the Earth's magnetic field, move towards its poles and, colliding with the ions of the upper atmosphere, the so-called ionosphere, give rise to a weak luminescence of the sky.

Image of a Northern Lights.
Courtesy Coelum Astronomia.

Beyond the particular manifestations we have mentioned, there is no doubt that the cycle of solar activity has its repercussions on the Earth, on its meteorological events, on the seasons, perhaps on the very physiology of plants and animals, including man. Indeed, it has been found that the thickness of the growth rings of plants is correlated to the cycle of solar activity, or at least to the historical series of Wolf numbers. Extrapolating this correlation towards the past, on the basis of the thickness of the growth rings of ancient or millenary fossil plants, in order to reconstruct the Wolf numbers for the periods in which such data are not available, we have found interesting correlations between the solar activity cycle and terrestrial glaciations.

This last aspect of the glaciations, made evident by the following graphs,

Vrrrr ... faster than light ... or wind?

L'sandstone or arenite

it is a sedimentary rock composed of granules of the average size of a sand. The granules can have various mineralogical compositions, depending on the area of ​​origin.

  • The quartz grains are the most abundant and give a high resistance to abrasion
  • It also contains other minerals that make up the sand, such as: feldspar, micas, calcite, zircon, apatite, monazite, magnetite and pyrite.
  • Cement of chemical origin through ion precipitation, the most common are: calcium carbonate, silica and iron oxide. This cement takes up the space between the granules and holds them together.

Based on the percentage of cement they can be classified into:

peliti - if the matrix is ​​present in percentages greater than 75%

greywacke - if the matrix is ​​present in percentages between 75% and 50%

subgrovacche - if the matrix is ​​present in percentages between 50% and 15%

areniti - if the matrix is ​​present in percentages lower than 15% and cement is present which lithifies the rock.



Stratified, only rarely massive.


Fine to coarse granules, recognizable to the touch, with dimensions of less than 2 mm.

Geotectonic formation environment

Location of discovery

Foredeep sandstones are widespread in the Northern Apennines and belong to formations that have different names moving from the Tyrrhenian Sea to the Adriatic Sea (Macigno delle Cinque Terre, Pseudomacigno, Macigno di Barga, Macigno dei Monti del Chianti, Sandstone of Monte Falterona, Arenarie of Monte Cervarola, Marnoso-Arenacea, Laga Formation).

Sailboats for those who don't want to give up the thrill of speed

There are two owners who are passionate about sailing, and knots, behind the last several of the Southern Wind Shipyard: two sister boats with pure lines programmed for regattas and cruises

If you think the sailboats can't give you the adrenaline of speed, be ready to change your mind. The Shipyard Southern Wind has launched two models of the same sail boat SW96: Sorceress and Seatius. Identical hull size declined in two elegant versions with very pure lines, united by a great feature: speed, in fact.

The first of the two sailing yacht is called Sorceress: signed by Nauta Design and Farr Y.D., it is 31.4 meters long, 6.95 meters wide and has a retractable keel and, for the first time, two rudder blades.

Result of the precise requests of a very demanding owner who wanted one super performance sailing yacht to be used for both coastal regattas both for those transoceanic, but also as a blue water cruiser (which for the impractical nautical terms we will translate with deep water cruises, away from the coast), the Sorceress sailboat it is characterized by balanced shapes and an exemplary deck line for elegance and cleanliness.

Its Canadian owner took part in the maiden voyage between Cape Town, South Africa - where the shipyard is based - and the Mediterranean. A one-of-a-kind ocean test, which all newly launched SW must complete and which, usually, is up to the technical team. In this case, the owner wanted to be on board to continue the path of constant collaboration undertaken with the shipyard and designers from the early stages of the project.

Jim Schmicker, vice president of Farr Yacht Design (the studio that designed the sailing boat together with the Italians of Nauta Design) said that Southern Wind asked for "a hull with a totally new style and image. The goal was to reach a level superior in terms of design, incorporating characteristics typical of the most performing yachts, such as the wide stern, two rudder blades, large sail area and stability, like those that have always distinguished our yachts: ease of handling, great balance, comfort in navigation and a solid feeling of seaworthiness. The performances, to satisfy the owner's requests, had to be higher than those of our previous projects for the shipyard ".

Low hydrodynamic resistance, high performance at all wind speeds, low weights even in the interiors are the principles that have animated the design research and which are also found in Seatius, the most recent sister boat, which differs in many details designed to satisfy its owner looking for a fast sailing boat to comfortably navigate around the world. Seatius is, in fact, more oriented to blue water cruises than to sailing competitions.

Despite the wide stern, the double wheelhouse and a respectable sail plan, Seatius is a very balanced boat that promotes comfortable sailing. The deck layout has been revolutionized in favor of a greater predisposition to comfort and safety during long ocean cruises. Unlike its sister Sorceress, Seatius has two side sunbathing areas divided by a central passage that runs on a single level so as to facilitate the movement of guests.

To increase comfort while sailing, a hard top protects the guest cockpit, while a bimini set with carbon structures protects the wheelhouse from sun, wind, rain or splashes of water.

This is also below deck sailing yacht it represents a different project than its sister Sorceress which has a four-cabin layout to accommodate larger crews during regattas. Seatius, on the other hand, has a three-cabin layout for greater comfort on a cruise. Finally, this configuration allows for a larger owner's suite and additional spaces dedicated to social activities for guests.

Both sailboats SW96 today they are ready to sail the seas at the speed of the wind!

  • 1 Description
  • 2 Classification
    • 2.1 Open cycle galleries
    • 2.2 Closed loop galleries
    • 2.3 Supersonic tunnels
    • 2.4 Dimensions
    • 2.5 Wind tunnels on the international and national scene
  • 3 Standard components of a subsonic wind tunnel
    • 3.1 Convergent
    • 3.2 Test chamber
    • 3.3 Divergent
    • 3.4 Curves
    • 3.5 Motor and fan
    • 3.6 Honeycomb type flow straightener
    • 3.7 Networks
  • 4 Applications
  • 5 Notes
  • 6 Bibliography
  • 7 Related items
  • 8 Other projects
  • 9 External links

The measurements that are made are typically measurements of: global and local velocities, measurements of pressure, temperature and forces exerted by the fluid on the body. In the wind tunnel, the so-called visualizations of the pressure, temperature and force fields that are established on the surface of the body or of the flow velocity field are also carried out. In the first case, the surface of the body is coated with particular substances sensitive to temperature, pressure or frictional forces.

In the second case, special tracers are used, such as coloring substances or fumes that allow to visualize the flow trend around the body. Another way to make the visualizations is to use wool threads attached to the surface of the body or to supports which are then appropriately moved to study particular areas of the field.

In the water tunnels, inks or opaque substances such as milk are also used as tracer, which has the advantage of not being polluting and cheap, as well as having a density very similar to that of water. In supersonic tunnels (but in general in all tunnels with compressible flow) the visualizations are made by exploiting the phenomenon of the refraction of light that passes through two substances with different densities.

The possibility of carrying out tests in the wind tunnel is based on the so-called principle of reciprocity, which states that from the point of view of the value of the physical quantities that are measured and the trend of the flows on the body, it is indifferent to move a body in a fluid. stationary or moving fluid around a stationary body.

Wind tunnels are divided into two main categories:

  • open cycle tunnels
  • closed loop tunnels

Another classification of wind tunnels distinguishes the latter with respect to the flow velocity in the test chamber:

  • incompressible subsonic tunnels if the Mach number of the current is between 0 and about 0.3
  • compressible subsonic tunnels if the Mach of the current is between about 0.3 and about 0.8
  • transonic tunnels if the Mach of the current is between 0.8 and 1.2
  • supersonic tunnels if the Mach of the current is between 1.2 and 5
  • hypersonic tunnels if the Mach of the current is greater than 5.

Open-loop galleries Edit

The open-cycle tunnels in the front part are generally composed of a mouth and a duct with a constant section (usually with a circular or rectangular section) where some devices are placed to control the quality of the incoming flow. This duct is followed by another one (with circular or rectangular section) of convergent type, which ends at the starting point of the test chamber which has a constant section and in which the model of the object is inserted. fluid dynamics in the test chamber the speed reached by the fluid is the highest, and precisely it must be at the maximum the design speed.

The test chamber is followed by a divergent duct (generally with a circular section) which is called divergent or diffuser. In a certain point of the divergent there is the motor, electric and faired, to which one or more fans are connected. The fans have the task of transferring the kinetic energy generated by the motor to the fluid, which is thus sucked into the test section. These components are followed by a further diverging duct for the compression of the flow and finally the expulsion section for the discharge of the flow into the external environment.

It is important to take into account that the fans, which as mentioned above have the task of transferring the kinetic energy supplied by the motor to the fluid, are placed downstream of the test chamber because they, in addition to supplying the fluid with the necessary kinetic energy, they also generate a series of vortices and downstream turbulence, the presence of which in the test chamber, and therefore on the model, would completely alter the value of the measurements made.

The disadvantages of an open tunnel are the noise and the apparent loss of energy that occurs due to the discharge of the accelerated fluid into the atmosphere. In reality, the latter disadvantage does not entirely occur. This is because if the flow outlet from the tunnel were joined with the inlet and a closed tunnel was created (as described below), there would be pressure losses (i.e. losses of energy of the fluid due to friction). This explains why open-cycle wind tunnels still exist and are used in the case of tests at modest speeds.

A much more consistent disadvantage than the one just described is given by the fact that the test chamber is closed and the pressure inside it is lower than the external one (see in this regard Bernoulli's theorem). Precisely because of this, the test chamber must be perfectly sealed to avoid infiltration of fluid from the outside which, being at higher pressure, would penetrate the chamber significantly altering the flow trend around the model and therefore the measurements made. .

Closed loop galleries Edit

Closed-loop tunnels have the same main components as open-loop tunnels. The only difference is that instead of being expelled outside the tunnel, the flow is recirculated inside. The advantages over the open solution are in the possibility of varying the characteristics of the fluid used (pressure, temperature, humidity, viscosity and so on) and of being able to use an open or semi-open test chamber, with considerable simplifications in terms of logistics in positioning the models to try. Closed-cycle tunnels must be equipped with heat exchangers and radiators capable of cooling the fluid which heats up consistently as it flows, with the possibility of altering the measurements made.

Supersonic Galleries Edit

Supersonic wind tunnels are primarily used to test the body's effects on a supersonic gas flow. In fact, Mach waves, compression waves, expansion waves and shock waves will be present in the flow around the body. Furthermore, the effect of the temperature of the fluid, which is no longer negligible, becomes a fundamental parameter for the simulations, as well as the variations in density which, already for speeds in the order of Mach 0.3, are no longer negligible.

Supersonic wind tunnels are basically of three types.

The first type foresees, to realize a supersonic flow, a large tank downstream of the test chamber, inside which a high vacuum is created. To carry out the test, a valve opens and the gas flows very quickly through a specially shaped duct, from the external environment towards the tank that is opening.

The second type of supersonic wind tunnel can be achieved by pressure filling a large reservoir upstream of the test chamber. In order to achieve the supersonic flow, a valve opens and the gas flows very quickly from the high pressure tank towards the test chamber.

These types of tunnels have the drawback of creating an intermittent flow since the tanks, once open, require a certain period of time to be emptied or refilled up to the initial conditions, thus making it impossible to carry out tests continuously.

This drawback can be overcome with the third type of supersonic tunnel, which can be built as a traditional closed tunnel of the subsonic type but with a supersonic flow in the test section. This involves enormous difficulties and very high costs since problems arise in the start-up phases of the tunnel which make it essential to use a variable geometry. Further differences, especially in terms of construction difficulties, complexity and cost increase, are represented by the need, in the case of a supersonic tunnel, to have a fluid drying system.

Remember that the behavior of a supersonic flow is radically different from that of a subsonic flow. In the case of a supersonic flow, in fact, if the section of the duct increases, the speed will tend to increase further and not to decrease as it happens instead in the case of a subsonic flow. This explains why some components of a supersonic tunnel are made differently than those of a subsonic tunnel. For example, in a supersonic tunnel a convergent-divergent duct will be placed before the test chamber and not just a convergent one because with the first convergent section a sonic flow will be created (unit Mach number) which then, upon entering the divergent duct, will become supersonic.

Size Change

Wind tunnels vary in size according to needs. Taking the test chambers for example, it ranges from tunnels with dimensions below one square meter section, to NASA's huge Ames wind tunnel with a 24x36 meter test chamber. The development of wind tunnels had an enormous acceleration during the Cold War thanks also to the enormous capital invested by the Russian and US governments. In these countries there are the largest and most powerful wind tunnels in the world, some capable of reaching Mach 25 for the tests of the re-entry spacecraft to Earth.

Naturally, the cost of construction and use of a wind tunnel varies according to its dimensions and the characteristics of both the flow that is created and the geometric characteristics of the tunnel itself. This is why open-cycle tunnels are the simplest and cheapest but have limited performance, while closed-cycle tunnels have higher costs both in terms of maintenance, construction and use, but a very high flow quality.

Wind tunnels on the international and national scene Edit

The wind tunnels, inspired by the achievements of Eiffel in Paris and Prandtl in Gottingen at the beginning of the twentieth century, have had widespread diffusion especially in the last 30 years (in Japan alone, in the last decade, 1000 have been built), giving resulting in multiple equipment distributed between the public sector, private industry and the academic world. They include three types of systems aimed, respectively, at the aeronautical sector, vehicle testing and civil and environmental measures.

Italy has numerous plants in the aeronautical and vehicular fields (just remember the Ferrari and Pininfarina tunnels and the plasma wind tunnel of the Italian Aerospace Research Center in Capua). Instead, it has only four tunnels for civil and environmental use - the first [1], built in Prato by CRIACIV, has medium dimensions, the second [2], built in Milan by CIRIVE at the Polytechnic, is the largest in Europe - insufficient to face the growing demands of the Italian market. The DICAT-DIFI [3] wind tunnel in Genoa, inaugurated in 2008, is characterized by complementary properties compared to the tunnels of Prato and Milan: in this way it will contribute to creating a network of laboratories, each with its own peculiar characteristics.

Convergent Change

The convergent serves to reduce the section and therefore increase the speed of the fluid while simultaneously reducing the level of turbulence and the thickness of the boundary layers on the walls. It is affected by a negative, and therefore favorable, pressure gradient (since there is more pressure at the beginning of the convergent and less at the end), therefore the problem of the separation of the boundary layer is not present, this allows a fairly rapid narrowing of the section. The contraction of the section (contraction ratio in English) is generally on the order of 6, very rarely is greater.

Test Chamber Edit

It is the place where the object to be tested is placed or where the measurements on the flow are carried out. It is of fundamental importance to reproduce as much as possible the real conditions in which the test object is working, it is also important to have a perfectly known flow in terms of Reynolds number, turbulence level, temperature, humidity and all the other variables that determine the characteristics of the flow. The test chamber is the point where the flow is at the highest speed, i.e. the maximum design speed of the tunnel, it is also of course the first component designed in the wind tunnel and must be large enough to accommodate the model to be tested so as not to run into in the blocking problem. This phenomenon is in practice the effect of the walls of the test chamber on the flow lines and on the velocity of the fluid in the vicinity of the object to be tested. In all tunnels there is the effect of blocking in theory, this is because if the body is found to operate, for example in the atmosphere, it will find itself operating in a "theoretically infinite" environment while in the wind tunnel it will always operate in an environment of a few square meters of section, so, if the model is small enough compared to the dimensions of the test chamber, it will be possible to reduce this effect or in any case correct the data obtained through mathematical models, specially calibrated for the particular geometry of the test chamber. test in question, implemented directly in the computers that process the data coming from the sensors in the tunnel.

Divergent Edit

It is the component that is located after the test chamber, imagining to follow the flow trend in the tunnel. It has several tasks: the first is to decrease the speed by increasing the section, the second function of the enlargement of the section is purely functional to rejoining the duct to the convergent in closed-loop tunnels. The slowing of the fluid after the test chamber is essential because by placing the motor, and therefore the fan or propeller in a low speed point, it is possible to install a less powerful motor and therefore less expensive and smaller. In the divergent there is a positive and therefore unfavorable pressure gradient, since the pressure at the end of the divergent is greater than the pressure value at the beginning. This leads to a thickening of the boundary layer and therefore the possibility arises that the very harmful phenomenon of the separation of the boundary layer occurs, this can be avoided with suitable devices for sucking the boundary layer or blowing or with very low divergence angles (maximum 2 ° or 3 °). Another solution is that of rapid divergers in which intermediate nets or bulkheads are used, capable of re-attaching the boundary layer to the wall of the divergent itself.

Curves Edit

They are used to change direction of the flow, they are composed of a duct with a suitably beveled angle and various airfoils arranged in a row that have the task of facilitating the flow to bend. The curves are affected by the phenomenon of separation of the boundary layer due to the fact that they have a high curvature, and to the fact that sometimes the outlet duct has a larger section than the inlet and therefore an adverse pressure gradient is present. However, thanks to the fact that arrays of profiles are used it is possible to avoid this annoying phenomenon. Curves by their nature tend to introduce axial vorticity which, however, can be minimized with the use of special devices and conformations of the curves themselves.

Motor and Fan Modification

Per sopperire alle perdite di carico o pressione (in parole povere di energia) che il fluido subisce lungo tutto il tragitto della galleria del vento, è necessario introdurre nel fluido una certa quantità di energia ogni secondo. È necessario quindi dotare la galleria del vento di un ventilatore comandato da un motore. Il motore (tipicamente un motore elettrico che ha il pregio di fornire una spinta quasi costante senza vibrazioni di un motore alternativo) può essere coassiale al ventilatore oppure può essere esterno.

Il caso del motore esterno è il migliore perché più schermato e quindi produce meno disturbo al fluido (in termini acustici) riscaldandolo anche di meno. Il ventilatore o i ventilatori sono composti da uno o una serie di eliche coassiali oppure affiancate nelle gallerie supersoniche le eliche sono dei veri e propri compressori assiali o centrifughi essi hanno il compito principale di trasferire l'energia cinetica fornita dal motore, al fluido sotto forma di energia di pressione. Si fa notare che, contrariamente a ciò che si pensa, il ventilatore o compressore, fornisce un salto di pressione e NON di velocità. Il trasferimento naturalmente non è ne adiabatico né isoentropico e questo vuol dire che una parte di energia cinetica va persa in calore.

Il ventilatore è posto a valle della camera di prova in una zona con una sezione elevata per ridurre la potenza necessaria da fornire. Esso deve inoltre essere messo più lontano possibile dall'ingresso della camera di prova secondo il percorso che deve fare il fluido, quindi l'ideale sarebbe quello di metterlo appena dietro il modello. Questa soluzione però è sfavorevole perché al termine della camera di prova la velocità è massima per due motivi principali: 1) in camera di prova l'obbiettivo è sempre quello di avere la velocità massima 2) alla fine della camera di prova lo strato limite risulta più spesso rispetto alla zona centrale della camera di prova, e questo causa una diminuzione della sezione effettiva vista dal fluido. Questo significa che la velocità ancora più elevata che all'inizio della camera di prova il ventilatore è quindi disposto alla fine del divergente, dove la velocità locale è più bassa.

Raddrizzatore di flusso tipo Honeycomb Modifica

The raddrizzatore di flusso [1] del tipo honeycomb [2] è composto da una serie di tubi di piccolo diametro (generalmente tra 5 e 7 mm) e lunghi circa una quindicina di centimetri. Questi tubicini sono disposti coassialmente al condotto nel quale si trovano (generalmente nella zona della galleria dove le velocità sono le più basse possibili per minimizzare le perdite di carico) e sono incollati l'uno all'altro a formare strutture, generalmente esagonali (simili ai nidi d'ape, da cui il nome), che ricoprono l'intera area della sezione dove sono posti. A titolo di esempio si cita che, in una galleria avente una sezione dove è presente l'honeycomb di 3,6 x 0,9 metri, sono disposti qualcosa come 78.000 tubicini di 7 mm di diametro. Il compito dell'honeycomb è quello di eliminare le componenti di velocità del flusso normali alle pareti e indirizzare il flusso unicamente e il più possibile in direzione dell'asse del condotto. Generalmente l'honeycomb o gli honeycomb sono posti poco prima dell'inizio del convergente e quindi appena prima della camera di prova.

Reti Modifica

Le reti hanno il compito di spezzare le strutture vorticose di grande scala e convertirle in strutture vorticose più piccole e più uniformi, si rende così il profilo di velocità più uniforme. Le reti hanno però, come già detto, il difetto di deviare la direzione principale del flusso, è perciò necessario posizionarle attorno ad elementi (l'honeycomb) capaci di “raddrizzare” il flusso. Sono componenti che tendono a sporcarsi e quindi, come l'honeycomb, necessitano di frequente manutenzione.

Tipicamente nelle gallerie del vento vengono provati aeroplani, automobili, camion, treni, elicotteri ed in misura minore, motociclette. Provano in galleria anche ciclisti, sciatori ed atleti per gare su ghiaccio come per i bob ed i pattinatori oppure gli atleti che si lanciano dagli aeroplani per stabilire il record di volo senza paracadute. Si provano anche edifici, ponti, generatori eolici, portaerei ed in generale tutti i veicoli e oggetti che si muovono nell'aria.

È possibile effettuare la prova di aeroplani (se la galleria è chiusa) variando le caratteristiche del fluido in termini di temperatura e umidità andando così a simulare le condizioni meteorologiche che l'aereo potrebbe incontrare nella sua vita operativa. Si possono così simulare l'effetto della formazione di ghiaccio sulle ali (che può causare lo stallo delle superfici portanti del velivolo e quindi la probabile caduta dell'aeroplano, o il possibile blocco dei comandi) o altri effetti delle perturbazioni meteorologiche sull'aerodinamica del velivolo.

La prova di edifici e strutture come i ponti viene fatta simulando l'andamento temporale e spaziale dei venti presenti nel luogo dove sorgeranno queste strutture. Le prove sugli edifici consentono anche di capire come, ad esempio, l'inquinamento da fumi di industrie va ad influire e ad interagire con i centri abitati.

Per l'utilizzo automobilistico si usano anche dei tappeti mobili capaci di simulare, nel modo più preciso possibile, l'effetto del suolo. Questi tappeti in pratica funzionano come degli enormi "tapis roulant" che viaggiano a svariate decine di metri al secondo. Recentemente si è parlato anche di tappeti mobili capaci di vibrare per simulare le sconnessioni e le imperfezioni del suolo, cosa questa molto importante nei veicoli a più alte prestazioni che viaggiano a pochi centimetri dal suolo per i quali è fondamentale studiare la stabilità degli strati limite e la loro interazione con il terreno.

Per quanto riguarda gli studi accademici oggi si effettuano principalmente degli esperimenti sugli strati limite, con studi di stabilità e turbolenza.

Naturalmente in galleria del vento si provano dei modelli degli oggetti reali realizzati in scala ridotta, e questo porta ad una serie di problemi praticamente insormontabili, almeno dal punto di vista teorico. La teoria della similitudine dinamica completa infatti impone delle condizioni sulle dimensioni dei modelli che costringerebbero di fatto a provare dei modelli non in scala ma in dimensione reale, con tutte le conseguenze che questo può portare, ad esempio in termini di costi di gestione e di realizzazione. Le simulazioni vengono fatte cercando di riprodurre sui modelli posti in galleria del vento alcuni parametri fisici caratteristici come i numeri di Reynolds, Eulero, Froude, Cauchy e Mach, dai quali dipendono alcuni fenomeni di rilevante interesse (Separazioni di strato limite, fenomeni di compressibilità, formazione di fenomeni ondosi nel campo aerodinamico, scambi termici tra strato limite e corpo). La teoria della similitudine, come accennato, impedisce che a parità di fluido si possano effettuare delle simulazioni con modelli in scala rispettando tutti i parametri sopra elencati contemporaneamente.

Questo problema è praticamente irrisolvibile e quindi nascono gallerie dalle dimensioni molto variabili, come detto in precedenza, che sono capaci di riprodurre contemporaneamente solo alcuni dei numeri sopra elencati, a seconda delle esigenze. Ad esempio, il modello di un aereo civile sarà studiato in una galleria subsonica capace di riprodurre i numeri di Reynolds che si raggiungono nelle reali condizioni di decollo e di atterraggio, mentre invece ci si affiderà ad una galleria transonica quando si dovrà indagare il suo comportamento in volo di crociera. Questo comporta che per avere informazioni precise e complete bisogna effettuare una serie di prove con modelli dalle dimensioni diverse ed in gallerie del vento diverse e questo fa crescere notevolmente i costi. Non solo: i dati ricavati dalle varie simulazioni non sono sempre completamente aderenti alla realtà ed in più non sono mai uguali e riproducibili se si cambia galleria del vento. Questo ha portato allo sviluppo di algoritmi e modelli matematici sempre più complessi e precisi che siano in grado di trasferire e interpretare correttamente i dati ricavati in galleria del vento al fine di prevedere nel modo più accurato possibile il comportamento dell'oggetto realizzato.

Si ponga l'attenzione sul fatto che questo problema è un problema rilevante nelle simulazioni in galleria del vento e non va quindi mai sottovalutato.

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