用控制容积法测定扁板的撞击力

The control volume method is a powerful tool in fluid engineering that replaces complex system details with a simplified free body diagram defined by an imaginary closed surface. Engineers use this approach to predict dominant loads on structures like wind turbine blades, boat sails, and airplane wings before pursuing detailed aerodynamic design. It provides an initial understanding of feasibility and dominant effects on a system.

Forces exerted on bodies by fluid flow result from changes in linear momentum of the flow around them. The control volume method applies conservation of linear momentum, where the sum of forces acting on the volume equals the rate of change of momentum inside it plus the net flux of momentum through the control surface. This principle enables engineers to calculate aerodynamic loads on structures.

Control volume analysis accounts for surface forces due to pressure or flow-induced shear, and body forces such as weight of solids and fluids inside the volume. The analysis also includes forces from volumetric effects like electromagnetic fields. All these forces, combined with momentum flux through the control surface, determine the net reaction force on the structure being analyzed.

The normal force is calculated by decomposing the reaction force into components perpendicular and tangent to the plate using the impingement angle. The normal load can be expressed using the plate span, jet width at vena contracta, the contraction ratio between jet exit and vena contracta, and the dynamic pressure at vena contracta to obtain the final theoretical expression.

An aerodynamic balance is used to directly measure lift and drag forces exerted by an impinging jet on a plate. The balance converts force measurements in volts to Newtons using calibration constants. A pressure transducer measures the pressure difference between the plenum and atmosphere, and a data acquisition system records force data at specified sampling rates to capture the aerodynamic loads.

Discrepancies between measured and theoretical forces typically range from 2% to 12.5% and vary with impingement angle. The control volume analysis assumes inviscid, non-dissipative changes in linear momentum, while direct measurements cannot avoid viscosity effects on the flow. For angles below 80 degrees, the method underestimates loads; above 80 degrees, it overestimates them.

Control volume analysis is used to design devices like Pelton blades that convert linear momentum into torque, with optimal geometry imposing 180-degree jet direction changes. Engineers also conduct downscaled model experiments in wind or water tunnels, using control volume analysis with upstream and downstream velocity measurements to predict prototype loads and feasibility before full-scale development.