Quanti tipi di galleria del vento esistono?

Che cos'è e qual è la differenza tra i tipi di galleria del vento

Che cos'è una galleria del vento?

Gallerie del vento sono grandi tubi in cui scorre l'aria, simulando le azioni di un oggetto che vola nell'aria o si muove sul terreno. Utilizziamo le gallerie del vento per testare i modelli in scala di aerei, veicoli spaziali, automobili e motociclette. Alcune gallerie del vento sono abbastanza grandi da contenere a grandezza naturale versions of vehicles or just any part of it. In this article, we’ll treat the wind tunnel classification and their principal components.

Aprilia Guzzi Wind tunnel

Galleria del vento: APRILIA GUZZI

Perché le gallerie del vento sono così importanti?

The possibility of carrying out tests in the wind tunnel is based on the so-called “principio di reciprocità”, which states that dal punto di vista del valore delle grandezze fisiche che vengono misurate e della portata sul corpoÈ indifferente muovere un corpo in un fluido fermo o muovere il fluido intorno a un corpo fermo. Quindi, invece di avere l'aria ferma e un oggetto che la attraversa a velocità, lo stesso effetto si otterrebbe se l'oggetto rimanesse fermo e l'aria si muovesse a velocità davanti a lui. In questo modo un osservatore fermo potrebbe misurare le forze aerodinamiche che gli vengono imposte.

Le prime gallerie del vento sono state inventate verso la fine del XIX secolo, agli albori della ricerca aeronautica, quando molti tentavano di sviluppare aerei e missili supersonici di successo.

In seguito, lo studio in galleria del vento si è fatto strada: gli effetti del vento su strutture artificiali or objects needed to be studied when buildings became tall enough (like skyscrapers) to present large surfaces to the wind, and the resulting forces had to be resisted by the building’s internal structure. Determining such forces was required before building codes could specify the required strength of such buildings and such tests continue to be used for large or unusual buildings.

wind tunnel types

Qual è la relazione tra i test in galleria del vento e la CFD (fluidodinamica computazionale)?

Ancora più tardi, i test nella galleria del vento è stato applicato alle automobiliNon tanto per determinare le forze aerodinamiche in sé, quanto piuttosto per stabilire come ridurre la potenza necessaria per spostare il veicolo su strada a una determinata velocità. In questi studi, l'interazione tra la strada e il veicolo gioca un ruolo significativo, e questa interazione deve essere presa in considerazione quando si interpretano i risultati dei test. In una situazione reale, la carreggiata si muove rispetto al veicolo ma l'aria è ferma rispetto alla carreggiata, mentre nella galleria del vento l'aria si muove rispetto alla carreggiata, mentre la carreggiata è ferma rispetto al veicolo di prova. Alcune gallerie del vento per test automobilistici hanno incorporato nastri mobili sotto il veicolo di prova nel tentativo di approssimare la condizione reale, e dispositivi molto simili sono utilizzati nei test in galleria del vento delle configurazioni di decollo e atterraggio degli aerei.

I progressi della fluidodinamica computazionale (CFD) su computer digitali ad alta velocità ha ridotto la richiesta di test in galleria del vento. Tuttavia, i risultati della CFD sono ancora non è completamente affidabile e le gallerie del vento vengono utilizzate per verificare le previsioni della CFD..

Correlazione CFD e Windtunnel

Correlazione CFD e Windtunnel

Due tipi principali di galleria del vento: a circuito aperto e a circuito chiuso.

Le gallerie del vento si dividono in due categorie principali:

  • aperto gallerie ciclabili (tipo EIFFEL)
  • chiuso gallerie ciclabili (gallerie di tipo PRANDTL)

Un'altra classificazione delle gallerie del vento distingue quest'ultima dalla velocità del flusso nella camera di prova:

  • incomprimibile subsonico se il numero di Mach corrente è compreso tra 0 e 0,3 circa;
  • comprimibile subsonico se il Mach corrente è compreso tra 0,3 e 0,8 circa;
  • tunne transonicols se il Mach corrente è compreso tra 0,8 e 1,2;
  • tunnel supersonici se il Mach corrente è compreso tra 1,2 e 5;
  • tunnel ipersonici se il Mach corrente è maggiore di 5.

As we are interested in motorsport area, we’ll discuss about Automotive Tunnels where the air speed is subsonic. These fall into two categories:

  • Flusso esterno I tunnel vengono utilizzati per studiare il flusso esterno attraverso lo chassis.
  • Climatic tunnels are used to evaluate the performance of door systems, braking systems, etc. under various climatic conditions.

For external flow tunnels various systems are used to compensate for the effect of the boundary layer on the road surface, including systems of moving belts under each wheel and the body of the car (5 or 7 belt systems) or one large belt under the entire car, or other methods of boundary layer control such as scoops or perforations to suck it away.

The measurements that are carried out are typically measurements of: global and local speeds, 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 you to view the flow pattern around the body. Another way to carry out the visualizations is to use woolen threads attached to the surface of the body or to supports which are then suitably moved to study particular areas of the field.

In water tunnels, inks or opaque substances such as milk are also used as tracers, which has the advantage of not being polluting and of costing little, 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 exploiting the phenomenon of the refraction of light that passes through two substances with different densities.

Descrizione dei tipi di galleria del vento: tipo aperto o chiuso.

Open loop tunnels.

The open cycle tunnels in the front are generally composed of a mouth and a duct with a constant section (usually with a section or circular or rectangular) where some devices are placed to control the quality of the incoming flow. This conduit is followed by another (circular or rectangular section) of a converging type, which ends at the starting point of the test chamber. The test chamber has a constant section and it is where the model of the object of which you want to study is inserted. In the test chamber the speed reached by the fluid is the highest, and precisely must be at maximum the design speed. If you want to see a working example of open wind tunnel, see this video.

How does a open wind tunnel works?

The test chamber is followed by a diverging duct (generally with a circular section) which is called a divergent or diffuser. At a certain point of the divergent there is the motor, electric and enclosed, 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 divergent conduit for the compression of the flow and finally the expulsion section for the discharge of the flow into the external environment.

Open Loop Wind Tunnel

Open Loop Wind Tunnel

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 located downstream of the test chamber; This is because, in addition to providing the fluid with the necessary kinetic energy, they also generate a series of vortices and turbulence downstream, 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 due to the discharge of the accelerated fluid into the atmosphere. In reality the latter disadvantage does not occur entirely. This is because if you join the exit of the flow from the gallery with the entrance and then create a closed gallery (as is described below) pressure drops (i.e. fluid energy losses due to friction) would occur. This explains why open-cycle wind tunnels still exist and are used for low speed tests.

Open Loop Windtunnel

A far more significant disadvantage than that 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 Bernoulli’s theorem in this regard). Precisely because of this, the test chamber must be perfectly sealed to prevent infiltration of fluid from the outside which, being at higher pressure, would penetrate the chamber, significantly altering the flow pattern around the model and therefore the measurements made.

Closed loop  wind tunnel.

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 compared to 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-loop tunnels must be equipped with heat exchangers and radiators capable of cooling the fluid which gradually heats up as it flows, with the possibility of altering the measurements made.

Closed Loop Wind Tunnel

Closed Loop Wind Tunnel

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