What Is a Drag Coefficient?

The resistance coefficient is often expressed as (C _d , C _x , C _w ) is a dimensionless quantity in fluid mechanics and is used to represent the resistance of an object in a fluid (such as water or air). The drag coefficient appears in the drag equation. A smaller drag coefficient indicates that the object is less subject to wind or fluid resistance. The drag coefficient is related to the shape of the object and its surface characteristics.

The coefficient of resistance refers to the ratio of the resistance experienced by an object (such as an airplane or a missile) to the pressure of air flow and the reference area.

Cx = X / (qS)

Where

Cx: resistance coefficient

X: resistance (resistance is in the same direction as the incoming velocity, and is positive backward)

q:

Resistance coefficient (Fr): The ratio of the pressure difference between the plugging agent flowing through the core and the pressure difference between the clear water flowing through the same core at the same flow rate. The formula is:

Fr = PG / PW
In the formula: PGdisplacement pressure difference of blocking agent inflow process at a certain flow rate.
Pwflow pressure difference of clear water inflow at a certain flow rate.

Resistance coefficient of rotating body

The resistance coefficient corresponds to an asymmetric object in the front and back. There is a front-end coefficient and a back-end coefficient. The object moves in the medium. The overall resistance is determined by these two coefficients.

Coefficient of drag of the car ^[1]

Wind erosion Gobi ^[2] is one of the aeolian landforms widely distributed in arid regions of China, and was one of the main sources of wind and sand activities and sandstorms in China during the geological period. During the development of the Gobi wind-eroded surface, the erodible material (sand grains) gradually decreased due to long-term wind erosion, while the non-erodible material (mainly gravel) was relatively enriched, forming an incorrodible gravel layer that protected the underlying material One-by-one Gobi wind erosion surface L? I. Although the inhibitory effect of Gobi wind erosion on wind and sand activity is well known, there are few quantitative studies on its aerodynamic behavior. The dimensionless drag coefficient reflects the obstruction of the airflow by the obstacles in the airflow. The drag coefficient on the surface of the Gobi can reflect the potential wind and sand activity and sand release intensity of the Gobi surface wind and sand activity interface, so it can be used to evaluate the aerodynamics of the Gobi wind erosion surface. The stability coefficient of specific surface conditions should be determined by field observations or simulation experiments. However, the field conditions are more complex. The resistance coefficient of the Gobi surface is not only related to the geometric characteristics of the surface gravel layer, but also affected by the topographic fluctuations. It is difficult to find an ideal observation place that is not affected by terrain fluctuations, so it is difficult to accurately distinguish the impact of gravel coverage and terrain fluctuations from the observation results. Furthermore, the gravels in the wild are complex in shape, difficult to accurately describe and uncontrollable geometric features. It is difficult to determine the quantitative relationship between the drag coefficient and the geometric characteristics of the gravel.

Gravel coverage increases the coefficient of resistance by 1.1-8 times. The increase factor depends on the size and coverage of the gravel. The resistance coefficient of the Gobi surface increases with the increase of the size and coverage of the gravel. For gravels of different sizes, the resistance The change of the coefficient with the coverage of the gravel obeys a similar law, that is, when the coverage is small, the process of increasing the resistance coefficient with the increase of the coverage of the gravel is more obvious, and the increase rate slows down with the increase of the coverage. When the degree is greater than 40%} 50%, the drag coefficient basically no longer increases with the increase of gravel coverage, indicating that the feedback of the Gobi bed to the airflow, that is, the blocking effect tends to stabilize, and the airbed interface reaches a relative equilibrium.

When the airflow passes the surface of the Gobi, its total resistance to the airflow can be decomposed into the resistance caused by the gravel and the resistance caused by the bare ground between the gravels, so the resistance coefficient can be expressed as

In the formula, C _dt is the total resistance coefficient, C _dg is the resistance coefficient generated by the gravel, and C _db is the resistance coefficient generated by the bare surface between the gravels.

The above formula can ideally be interpreted as: when the drag coefficient of the Gobi wind erosion surface tends to be constant, it is constant. It can be considered that the gravels added between the gravels that have been stabilized on the Gobi wind erosion surface will all be under the protection of the existing gravels, and the drag coefficient generated by the newly added gravels will tend to zero. The maximum resistance coefficient) can be used as a criterion for the aerodynamic stability of the Gobi wind erosion surface. For the various gravels tested, 40% -50% is the critical coverage of the mature and stable Gobi wind erosion surface.