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 2-تقرير عن center of pressure

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تاريخ التسجيل : 21/09/2008

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مُساهمةموضوع: 2-تقرير عن center of pressure   2-تقرير عن center of pressure Icon_minitime2008-09-23, 7:03 pm

Forces on Submerged Surfaces in Static Fluids
We have seen the following features of statics fluids

  • Hydrostatic vertical pressure distribution
  • Pressures at any equal depths in a continuous fluid are equal
  • Pressure at a point acts equally in all directions (Pascal's law).
  • Forces from a fluid on a boundary acts at right angles to that boundary.

Objectives:
We will use these to analyse and obtain expressions for the forces on submerged surfaces. In doing this it should also be clear the difference between:

  • Pressure which is a scalar quantity whose value is equal in all directions and,
  • Force, which is a vector quantity having both magnitude and direction.

1. Fluid pressure on a surface


Pressure is defined as force per unit area. If a pressure p acts on a small area 2-تقرير عن center of pressure Img00117 then the force exerted on that area will be
2-تقرير عن center of pressure Img00118
Since the fluid is at rest the force will act at right-angles to the surface.
General submerged plane
Consider the plane surface shown in the figure below. The total area is made up of many elemental areas. The force on each elemental area is always normal to the surface but, in general, each force is of different magnitude as the pressure usually varies.
2-تقرير عن center of pressure Img00119
We can find the total or resultantforce, R, on the plane by summing up all of the forces on the small elements i.e.
2-تقرير عن center of pressure Img00120
This resultant force will act through the centre of pressure, hence we can say
If the surface is a plane the force can be represented by one single resultant force,
acting at right-angles to the plane through the centre of pressure.

Horizontal submerged plane
For a horizontal plane submerged in a liquid (or a plane experiencing uniform pressure over its surface), the pressure, p, will be equal at all points of the surface. Thus the resultant force will be given by
2-تقرير عن center of pressure Img00121
Curved submerged surface
If the surface is curved, each elemental force will be a different magnitude and in different direction but still normal to the surface of that element. The resultant force can be found by resolving all forces into orthogonal co-ordinate directions to obtain its magnitude and direction. This will always be less than the sum of the individual forces,2-تقرير عن center of pressure Img00122.
2. Resultant Force and Centre of Pressure on a submerged plane surface in a liquid.


2-تقرير عن center of pressure Img00123
This plane surface is totally submerged in a liquid of density 2-تقرير عن center of pressure Img00124 and inclined at an angle of 2-تقرير عن center of pressure Img00125 to the horizontal. Taking pressure as zero at the surface and measuring down from the surface, the pressure on an element 2-تقرير عن center of pressure Img00126 , submerged a distance z, is given by
2-تقرير عن center of pressure Img00127
and therefore the force on the element is
2-تقرير عن center of pressure Img00128
The resultant force can be found by summing all of these forces i.e.
2-تقرير عن center of pressure Img00129
(assuming 2-تقرير عن center of pressure Img00130 and g as constant).
The term 2-تقرير عن center of pressure Img00131 is known as the 1st Moment of Area of the plane PQ about the free surface. It is equal to 2-تقرير عن center of pressure Img00132 i.e.
2-تقرير عن center of pressure Img00133
where A is the area of the plane and 2-تقرير عن center of pressure Img00134is the depth (distance from the free surface) to the centroid, G. This can also be written in terms of distance from point O ( as 2-تقرير عن center of pressure Img00135)
2-تقرير عن center of pressure Img00136
Thus:
The resultant force on a plane
2-تقرير عن center of pressure Img00137
This resultant force acts at right angles to the plane through the centre of pressure, C, at a depth D. The moment of R about any point will be equal to the sum of the moments of the forces on all the elements 2-تقرير عن center of pressure Img00138 of the plane about the same point. We use this to find the position of the centre of pressure.
It is convenient to take moments about the point where a projection of the plane passes through the surface, point O in the figure.
2-تقرير عن center of pressure Img00139
We can calculate the force on each elemental area:
2-تقرير عن center of pressure Img00140
And the moment of this force is:
2-تقرير عن center of pressure Img00141
2-تقرير عن center of pressure Img00142 are the same for each element, so the total moment is
2-تقرير عن center of pressure Img00143
We know the resultant force from above 2-تقرير عن center of pressure Img00144, which acts through the centre of pressure at C, so
2-تقرير عن center of pressure Img00145
Equating gives,
2-تقرير عن center of pressure Img00146
Thus the position of the centre of pressure along the plane measure from the point O is:
2-تقرير عن center of pressure Img00147
It look a rather difficult formula to calculate - particularly the summation term. Fortunately this term is known as the 2nd Moment of Area , 2-تقرير عن center of pressure Img00148, of the plane about the axis through O and it can be easily calculated for many common shapes. So, we know:
2-تقرير عن center of pressure Img00149
And as we have also seen that 2-تقرير عن center of pressure Img001501st Moment of area about a line through O,
Thus the position of the centre of pressure along the plane measure from the point O is:
2-تقرير عن center of pressure Img00151
and depth to the centre of pressure is
2-تقرير عن center of pressure Img00152
How do you calculate the 2nd moment of area?


To calculate the 2nd moment of area of a plane about an axis through O, we use the parallel axis theorem together with values of the 2nd moment of area about an axis though the centroid of the shape obtained from tables of geometric properties.
The parallel axis theorem can be written
2-تقرير عن center of pressure Img00153
where 2-تقرير عن center of pressure Img00154 is the 2nd moment of area about an axis though the centroid G of the plane.
Using this we get the following expressions for the position of the centre of pressure
2-تقرير عن center of pressure Img00155
(In the examination the parallel axis theorem and the 2-تقرير عن center of pressure Img00156 will be given)
The second moment of area of some common shapes.


The table blow given some examples of the 2nd moment of area about a line through the centroid of some common shapes.

ShapeArea A2nd moment of area, 2-تقرير عن center of pressure Img00157, about
an axis through the centroid
Rectangle
2-تقرير عن center of pressure Img00158


2-تقرير عن center of pressure Img00159


2-تقرير عن center of pressure Img00160
Triangle
2-تقرير عن center of pressure Img00161


2-تقرير عن center of pressure Img00162


2-تقرير عن center of pressure Img00163
Circle
2-تقرير عن center of pressure Img00164


2-تقرير عن center of pressure Img00165


2-تقرير عن center of pressure Img00166
Semicircle
2-تقرير عن center of pressure Img00167


2-تقرير عن center of pressure Img00168


2-تقرير عن center of pressure Img00169
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