Drag (physics)
The flow resistance is the physical size, referred to in the fluid dynamics, the force that opposes the fluid as a medium movement.
A body which moves relative to a gaseous or liquid medium undergoes a flow resistance, the relative speed opposite to the force acting.
If a person moves (for example, a jogger ) or an object (eg an airplane ) in the air or through the air, so one speaks also of air resistance in hydrodynamic problems in the water of water resistance.
Forces to flow around the body
On the surface of a body in flow, the flow from place to place, and pressure is applied shear stress of (normal voltage). Integration takes the pressure and shear stress over the whole surface results at the resulting force exerted by the flow on the body.
This resultant force has a particular direction in space. And the component force which is towards the flow direction, the resistance force. In addition to the resistance of other components are the buoyancy force and the lateral force.
For motor vehicles, these forces are measured in the wind tunnel. Here, it is common to specify the components of force with respect to a vehicle-fixed coordinate system.
Components of the flow resistance
It is the physical parameters such as pressure and shear stress acting on the surface of a body, and thus can contribute to flow resistance. Accordingly, the flow resistance in a pressure resistance and friction resistance can be split. Depending on the shape of the flow around the body, the pressure resistance or the frictional resistance may predominate.
According to the present case, it proves for viewing and calculation to be advantageous to treat certain effects that occur in the flow around the body separately. This is the background for the interference resistance, induced resistance and the characteristic impedance.
Pressure drag ( form drag )
The pressure resistance follows from the distribution of pressure ( normal stress ) to a body. The pressure in the separation region at the rear of bodies is lower than that of the stagnation point. The effective area of this resistance is, the projected area in the direction of flow.
Frictional resistance ( sheet resistance )
The frictional resistance is a result of friction, so the viscous momentum exchange. The frictional resistance due to the shear stresses that occur on the surface of the body by the flow sweeps across the surface.
Interference resistance
The interference resistance describes the fluidic resistor size that occurs when previously are completely independent body to flow spaced flow bodies. It is defined as the difference between the overall resistance of the device and the sum of the resistance of the individual components or groups of components after assembly. Structurally you will always strive for a negative interference resistance. An example is an airplane fuselage and aircraft wings before assembly and after assembly. The sum of the resistances of the components of the wing and fuselage is less than the total resistance after assembly. Qualitatively, the interference resistance acting against the inflow direction of the air component of force to a flow body which is caused by the mutual interference of the various parts of the aircraft caused by vortex or by superimposing the interfaces at the corners.
Induced resistance
The induced resistance occurs when lift-producing bodies, such as the wings of airplanes on. In such bodies the flow around produces a pressure difference between the top and the bottom. At the lateral edge of the body compensates for the pressure difference, resulting in circulation of the fluid mass on the side edge. It is formed from a wake vortex. The kinetic energy, which is fed to the fluid flow around the object is lost.
Impedance
The characteristic impedance occurs at flow around bodies moving at supersonic or transonic speed. At body edges that are contrary to the oncoming flow inclined occurs an increase in pressure during a pressure decrease occurs at the edges, which are averse to the incident flow. This pressure results in a force directed counter to the movement.
Dependence of the flow resistance
The drag force FW of a body in a particular location is dependent on the flow velocity v, density ρ and the viscosity ( viscosity ) η of the fluid, as well as the geometric dimensions ( a characteristic length ) L of the body.
This relationship, which has five variables can also be formulated by means of two non-dimensional similarity measures with the aid of a dimensional analysis on the Buckinghamschen Π theorem. This similarity metrics are the drag coefficient Cd and the Reynolds number Re, which are defined as
The variable A is a reference surface, which must be defined. Usually, the end face of the body is used as a reference surface, but the wing airfoil surface is taken.
The physical relationship can be described thus in the form
The drag force FW is proportional to the product of drag coefficient and reference area which is called drag area. Obtained from the drag force
Factor: is referred to as back pressure.
For many practical applications, the function of the Reynolds number is negligible. Then the drag coefficient is set as a constant value, so that the resistance increases quadratically with speed. This is, for example, when air resistance of motor vehicles the case. For a comparison of the flow resistance of different vehicles, the drag area is the decisive criterion.
Laminar flow
For laminar flow, the flow resistance is caused only by the internal friction of the medium. Is the dynamic viscosity of the medium, the following applies Stokes' law for spherical bodies of radius
Turbulent flow
In a turbulent flow, the flow resistance can be determined only by experiment, or by complicated numerical calculation, eg by means of finite element methods approach.
For motor vehicles, it can be assumed in the relevant velocity range of turbulent flow. In the modern automobile is the drag coefficient, drag coefficient, is of great importance. He may be in the optimal case 0.09, with a coach, it rises to 0.6.