# Tag Info

## Hot answers tagged reynolds-number

12

In still air every boundary layer starts laminar. How soon it transitions to a turbulent boundary layer depends on: the local Reynolds number, the pressure gradient, wing sweep and disturbances like bugs, rivet heads or turbulators. Flat plate flow (without pressure changes) normally transitions at a Reynolds number of around 400,000. If the flow is ...

9

Reynolds number is basically the ratio between the viscous and inertial forces acting on the system. At low Reynolds numbers, the viscous force is dominant compared to the inertial force and it is the other way at higher Reynolds numbers. The effect of the viscosity can be considered analogous to the dampers in a car suspension. If the damping is good, the ...

7

The aerodynamic coefficients $c_l,c_d,c_m$ are in general functions of the angle of attack $\alpha$, Reynolds $Re$ and Mach number $Ma$. For your experiments, since you are operating in a very low $Re$ regime, assuming incompressible flow, your lift, drag, moment curves will largely depend on viscous phenomena (i.e. $Re$ number). Generally, as $Re$ number ...

6

Yes, it does - skin friction increases with an increasing altitude. The mechanism is linked more to temperature than to density, but the reasoning in your question is correct. There might be a few cases where viscous drag increases with Reynolds number (like laminar airfoils which lose their laminar bucket when the Reynolds number goes up), but in general ...

5

No, a higher Reynolds number only signifies a lower ratio of viscous to inertial forces. Since you increase the Reynolds number by either increasing speed or length, both will drive up the inertial forces, making the friction forces relatively lower. Let's do a Gedankenexperiment: Consider an infinitely thin flat plate which moves through air in its ...

5

Recall the definition of the Reynolds number: Where ρ is the density, v is the velocity, μ is the viscosity of air and l is the characteristic length. The characteristic length is typically the chord length of an airfoil but can also mean the overall length of the airframe. When we scale down a model, we are decreasing l. Decreasing l while holding ...

4

It would be great if we could match all parameters at once. But that would require a tunnel large enough to accommodate the full size airplane and powerful enough to match the actual flight speed. The reality is that we cannot in most cases. We have some answers here on windtunnel testing. They should cover the basics: How are wind tunnels used with scale ...

4

Definitely use the local Reynolds number on each of the flaps! The flap will only work properly if it is allowed to produce its own, fresh boundary layer. If instead it inherits the slow, thick boundary layer of the wing or flap ahead of it, the flow would soon separate and the flap would loose most of its effectiveness. Note that a Reynolds number is ...

3

The Benedek 10355 clearly stalls with separation starting from the trailing edge. There is some discussion whether a laminar-to-turbulent transition due to rising pressure will always involve a laminar separation bubble, however short. But that bubble does not constitute a leading edge stall. XFOIL is not meant for flow simulation deep into the stall region. ...

3

The laminar "bubble" on the bottom is caused by the wavy contour. There is a pressure rise at about 25% on the lower side which causes the boundary layer to thicken, but the subsequent flat pressure profile keeps the boundary layer laminar until it separates at the trailing edge. Here is the plot from XFOIL and I placed a green circle over the spot:...

3

Near as I know, the key to designing good re-entry vehicles or shielding is large flat plates, primarily because the design ensures the energy transfer from kinetic and potential energy of the craft to thermal energy occurs mostly in heating the air flowing around the shape rather than the craft itself. It sounds kind of counter intuitive, but doing this ...

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Yes, the boundary layer that surrounds the airfoil is known to decrease its thickness as the Reynolds number increases; its relative thickness to the characteristic length (the cord for example) scales as $Re^{-1/2}$ in steady flow, and the polar curve becomes more horizontal when Re increases. Meanwhile, lift coefficient increases just a little bit (check ...

2

It is extremely expensive to build a wind tunnel that can be both pressurized and handle high-velocity flow. Such large-section wind tunnels do exist, but to maintain high pressure during a test run they require enormous pressure tanks which take ~hours to pump up to the required pressure, which can then be maintained for only ~seconds during a full blowdown ...

2

Golfballs are dimpled because the resulting turbulent boundary layer can follow the contour of the ball much better. Subsonic turbulent boundary layers have more drag than laminar ones, but fast separating boundary layers create high pressure drag, which more than compensates for the higher boundary layer drag. So that is all great for subsonic golfballs. ...

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An intuitive way to look at it, is that an increase in Reynolds number can be interpreted as a decrease in viscosity taking all other components constant. $\mathrm{Re} = \frac{\rho u L}{\mu} = \frac{u L}{\nu}$ One can intuitively feel that an object moving through a less vicious fluid will experience less drag. While the lift induced by momentum change in ...

2

Good question. The thing is in the velocity profiles for laminar and turbulent boundary layers. Lets look at the picture below. The profiles are little different. The turbulent profile is "fatter", or fuller, than the laminar profile. For the turbulent profile, from the outer edge to a point near the surface, the velocity remains reasonably close to the ...

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From the equation that defines it, you can see that its value depends on some properties of the fluid (air), the speed that an object moves through the fluid, and the length of the object. Nothing else. For an airplane wing, the length, aka "characteristic linear dimension," is conventionally chosen to be the chord of the wing. The wingspan doesn't affect ...

1

Putting slats and flaps on a 10 cm model is extreme, but would add a touch of realism for the serious hobbyist. At which point in scale do slats and flaps become unnecessary? We turn to the lift equation: $Lift$ = Air density x Area x Lift Coefficient x Velocity$^2$ $Weight$= $Lift$ for steady state flight so we derive: $Weight/Area$ = Rho x Lift ...

1

Flow at the forward stagnation point of an airfoil is laminar, because the local velocity will be zero. The boundary layer originating form the stagnation point will usually not remain laminar for long, but some airfoils like the NACA 6 series were optimized to conserve this laminar flow as far down the chord as possible. A laminar boundary layer is thinner ...

1

The basic reason is that the energy to create the turbulence comes in the first place from the forward motion of the aircraft or air vehicle.

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