# Air colliding with surface, local effects?

When a fluid, like air, is moving at some velocity and "hits" or collides with a surface:

• If the surface is at an angle, the fluid will tend to be propelled in a similar direction (anything else?).
• If the surface is perpendicular, the fluid will tend to kind of splash out in all directions (anything else?).

For generally turbulent, laminar, or mixed flow conditions? The surface has mass and is relatively stationary for the purposes of this question.

In each of those scenarios, what are the (qualitative) local effects of physical properties like pressure, density, etc. for both turbulent or laminar flow conditions? Does the air-like fluid compress into a denser form and the local pressure increase where the fluid is coming in contact with the surface?

• What kind of fluid/flow are you talking about? Compressible or incompressible? Turbulent or laminar? Viscous or inviscid? Is the surface rigid, or deformable? The answer varies greatly depending on the specifics.
– Paul
Jan 27 '16 at 5:33
• I haven't thought about this stuff in a while, I'd forgotten about all of that. thank you. I will keep editing it until it is satisfactory
– Zero
Jan 28 '16 at 5:26

When the velocity of the air is less than about Mach 0.3, there is hardly any change in pressure or density as it impacts the surface. For flows greater than $M=0.3$, you can calculate the stagnation pressure, density, and temperature with the following equations:

$$\frac{p_0}{p}=\left( 1+ \frac{\gamma - 1}{2}\cdot M^2 \right)^\frac{\gamma}{\gamma -1}$$ $$\frac{\rho_0}{\rho}=\left( 1+ \frac{\gamma - 1}{2}\cdot M^2 \right)^\frac{1}{\gamma -1}$$ $$\frac{T_0}{T}=1+ \frac{\gamma - 1}{2}\cdot M^2$$

Where $M$ is the mach number and $\gamma$ is the heat capacity ratio. The subscript $0$ is the stagnation value and no subscript is the free-stream value. The fluid "stagnates" at the point of impact with the surface, at which point its kinetic energy transforms into thermal and pressure energy, hence the increase in pressure, density, temperature, etc.

Beyond the stagnation zone, its not so easy to calculate the flow field. I ran a test case in ANSYS Fluent with an 80mm pipe shooting air at a solid wall 100mm away at a velocity of 10 m/s. The Reynolds number is about 28,000 for these conditions. I used the Reynolds stress model to calculate turbulence. The pressure and density fields are mostly constant as should be expected for this velocity. The velocity field is plotted in the image below. You can see the air make several vortexes along the wall. Hope this gives you some things to think about for your problem.

• omg so awesome. definitely gives me the information to gain insight. I will accept your answer for now since i cant even up vote yet.
– Zero
Feb 2 '16 at 3:03