Want to calc pressure of airflow on a surface

[sciguyjim]

I want to calculate the back-pressure caused by an airflow hitting a concave surface (high drag) perpendicular to the airflow. I'm starting with airflow in CFM through a tube, a rough measurement of the area impacted in square inches at the end of the tube, and the tube diameter. I did this once before, years ago but I can't recall how. I want to put a shaped covering over the concavity to smooth the airflow around it. I'd appreciate some help, thanks.

 

[poetix99]

Do I understand this correctly?:

You want to calculate the back-pressure induced in a tube when you place an obstruction near the tube discharge, and you want to be able to change (reduce) that back pressure by smoothing the obstruction.

(If not, please disregard all that follows.)

The concave surface (covered or uncovered) must be very close to the tube discharge end if it is to have any effect upon the back pressure in the first place. More than a few diameters downstream, and the tube won't "know" that any obstruction is there, regardless of its shape.

I don't know how you can accurately predict this (without some CFD s/w). But if you can measure the back-pressure in one of the obstructed configurations (or the flow in the free-flow case and one of the obstructed cases), and if you know the drag coefficients of each shape, you could then guesstimate the back pressure for the other obstruction.

 

[sciguyjim]

Hi Poetix99,
You understand correctly. The concave obstruction is just inside the end of the tube. The air can be thought of as just flowing in a tube past this obstruction. I'm not concerned here with what happens to the air after it passes this point. I am however trying to make the airflow as smooth as I can up to and including this point by reducing surface irregularities inside the tube and rounding over the high drag (Cd>1) concave surface the air is forced to run into. I intend to put a low drag (Cd<1) &quot;nosecone&quot;-type of covering over the concavity to reduce the turbulence of the air flowing over and around it. I just wanted to figure out some value for how much drag (backpressure) I eliminated by installing the &quot;nosecone&quot;.
Some numbers: Tube diameter: 3&quot;x5&quot;. Concavity: about 3 to 4 square inches in area inset about 1&quot; from back end of 3&quot;x5&quot; tube. Airflow is modulated by valves at the &quot;back wall&quot; of the 3&quot;x5&quot; tube. Air comes down tube, runs into concave surface, goes around this wall and through adjustable valves in back wall. Air temp: about 100°F. Air speed I figure is up to 167 mph. Quantity of airflow past concavity is up to 600 CFM. Basically all I'm doing here is reducing the drag induced by the concavity by installing the &quot;nosecone&quot;. I have no means of taking airflow measurements, I'm just using the values I gave here. Do you think it is possible to calculate a backpressure just knowing what I've given here and the approximate difference in drag coefficient between the concavity and nosecone configurations? Thanks.