A drumlin , from the Irish word droimnín ("little ridge"), first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine . Assemblages of drumlins are referred to as fields or swarms; they can create a landscape which is often described as having a 'basket of eggs topography'.
41-446: Tump means a hillock, mound, barrow or tumulus. The Welsh words twmp and Twmpath may be related. Although some may appear similar to glacial drumlins , for the most part they are man-made, e.g. remains from mineral extraction, burial mounds (tumuli and especially bowl barrows ) or motte-and-bailey castle mounds. The following geographical features in the UK are referred to using
82-951: A Newtonian flow; in fact it can be expressed as ( τ x x τ x y τ y x τ y y ) = ( x 0 0 − t ) ⋅ ( ∂ u ∂ x ∂ u ∂ y ∂ v ∂ x ∂ v ∂ y ) , {\displaystyle {\begin{pmatrix}\tau _{xx}&\tau _{xy}\\\tau _{yx}&\tau _{yy}\end{pmatrix}}={\begin{pmatrix}x&0\\0&-t\end{pmatrix}}\cdot {\begin{pmatrix}{\frac {\partial u}{\partial x}}&{\frac {\partial u}{\partial y}}\\{\frac {\partial v}{\partial x}}&{\frac {\partial v}{\partial y}}\end{pmatrix}},} i.e., an anisotropic flow with
123-523: A drumlin as it is repositioned and deposited. A hypothesis that catastrophic sub-glacial floods form drumlins by deposition or erosion challenges conventional explanations for drumlins. It includes deposition of glaciofluvial sediment in cavities scoured into a glacier bed by subglacial meltwater, and remnant ridges left behind by erosion of soft sediment or hard rock by turbulent meltwater. This hypothesis requires huge, subglacial meltwater floods, each of which would raise sea level by tens of centimeters in
164-491: A few weeks. Studies of erosional forms in bedrock at French River, Ontario, Canada, provide evidence for such floods. The recent retreat of a marginal outlet glacier of Hofsjökull in Iceland exposed a drumlin field with more than 50 drumlins ranging from 90 to 320 m (300–1,050 ft) in length, 30 to 105 m (100–340 ft) in width, and 5 to 10 m (16–33 ft) in height. These formed through
205-410: A generic tensorial identity: one can always find an expression of the viscosity as function of the flow velocity given any expression of the shear stress as function of the flow velocity. On the other hand, given a shear stress as function of the flow velocity, it represents a Newtonian flow only if it can be expressed as a constant for the gradient of the flow velocity. The constant one finds in this case
246-463: A length to width ratio of between 1.7 and 4.1 and it has been suggested that this ratio can indicate the velocity of the glacier. That is, since ice flows in laminar flow, the resistance to flow is frictional and depends on area of contact; thus, a more elongated drumlin would indicate a lower velocity and a shorter one would indicate a higher velocity. Drumlins and drumlin swarms are glacial landforms composed primarily of glacial till . They form near
287-609: A particle in a fluid passes through the fringes, a receiver detects the reflection of the fringe pattern. The signal can be processed, and from the fringe angle, the height and velocity of the particle can be extrapolated. The measured value of the wall velocity gradient is independent of the fluid properties, and as a result does not require calibration. Recent advancements in the micro-optic fabrication technologies have made it possible to use integrated diffractive optical elements to fabricate diverging fringe shear stress sensors usable both in air and liquid. A further measurement technique
328-552: A progression of subglacial depositional and erosional processes, with each horizontal till bed within the drumlin created by an individual surge of the glacier. The above theory for the formation of these Icelandic drumlins best explains one type of drumlin. However, it does not provide a unifying explanation of all drumlins. For example, drumlin fields including drumlins composed entirely of hard bedrock cannot be explained by deposition and erosion of unconsolidated beds. Furthermore, hairpin scours around many drumlins are best explained by
369-411: A solid boundary will incur a shear stress at that boundary. The no-slip condition dictates that the speed of the fluid at the boundary (relative to the boundary) is zero; although at some height from the boundary, the flow speed must equal that of the fluid. The region between these two points is named the boundary layer . For all Newtonian fluids in laminar flow , the shear stress is proportional to
410-399: A stereonet, scientists are able to see if there is a correlation between each clast and the overall orientation of the drumlin: the more similar in orientation and dip of the clasts throughout the drumlin, the more likely it is that they had been deposited during the formation process. If the opposite is true, and there doesn't seem to be a link between the drumlin and the till, it suggests that
451-423: Is also known as Zhuravskii shear stress formula after Dmitrii Ivanovich Zhuravskii , who derived it in 1855. Shear stresses within a semi-monocoque structure may be calculated by idealizing the cross-section of the structure into a set of stringers (carrying only axial loads) and webs (carrying only shear flows ). Dividing the shear flow by the thickness of a given portion of the semi-monocoque structure yields
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#1732787825059492-667: Is named dynamic viscosity . For an isotropic Newtonian flow, it is a scalar, while for anisotropic Newtonian flows, it can be a second-order tensor. The fundamental aspect is that for a Newtonian fluid, the dynamic viscosity is independent of flow velocity (i.e., the shear stress constitutive law is linear ), while for non-Newtonian flows this is not true, and one should allow for the modification τ ( u ) = μ ( u ) ∇ u . {\displaystyle {\boldsymbol {\tau }}(\mathbf {u} )=\mu (\mathbf {u} ){\boldsymbol {\nabla }}\mathbf {u} .} This no longer Newton's law but
533-413: Is non-Newtonian since the viscosity depends on flow velocity. This non-Newtonian flow is isotropic (the matrix is proportional to the identity matrix), so the viscosity is simply a scalar: μ ( u ) = 1 u . {\displaystyle \mu (u)={\frac {1}{u}}.} This relationship can be exploited to measure the wall shear stress. If a sensor could directly measure
574-412: Is nonuniform (depends on space coordinates) and transient, but is independent of the flow velocity: μ ( x , t ) = ( x 0 0 − t ) . {\displaystyle {\boldsymbol {\mu }}(x,t)={\begin{pmatrix}x&0\\0&-t\end{pmatrix}}.} This flow is therefore Newtonian. On the other hand, a flow in which
615-416: Is parallel to the direction of movement of the glacier at the time of formation. Inspection of aerial photos of these fields reveals glacier's progress through the landscape. The Múlajökull drumlins of Hofsjökull are also arrayed in a splayed fan distribution around an arc of 180°. This field surrounds the current lobe of the glacier and provide a view into the past, showing the previous extent and motion of
656-409: Is that of slender wall-mounted micro-pillars made of the flexible polymer polydimethylsiloxane , which bend in reaction to the applying drag forces in the vicinity of the wall. The sensor thereby belongs to the indirect measurement principles relying on the relationship between near-wall velocity gradients and the local wall-shear stress. The electro-diffusional method measures the wall shear rate in
697-422: Is the dynamic viscosity , u is the flow velocity, and y is the distance from the wall. It is used, for example, in the description of arterial blood flow , where there is evidence that it affects the atherogenic process. Pure shear stress is related to pure shear strain , denoted γ , by the equation τ = γ G , {\displaystyle \tau =\gamma G,} where G
738-517: Is the shear modulus of the isotropic material, given by G = E 2 ( 1 + ν ) . {\displaystyle G={\frac {E}{2(1+\nu )}}.} Here, E is Young's modulus and ν is Poisson's ratio . Beam shear is defined as the internal shear stress of a beam caused by the shear force applied to the beam: τ := f Q I b , {\displaystyle \tau :={\frac {fQ}{Ib}},} where The beam shear formula
779-504: Is the component of stress coplanar with a material cross section . It arises from the shear force , the component of force vector parallel to the material cross section. Normal stress , on the other hand, arises from the force vector component perpendicular to the material cross section on which it acts. The formula to calculate average shear stress τ or force per unit area is: τ = F A , {\displaystyle \tau ={F \over A},} where F
820-851: Is the dynamic viscosity of the flow. Considering a 2D space in Cartesian coordinates ( x , y ) (the flow velocity components are respectively ( u , v ) ), the shear stress matrix given by ( τ x x τ x y τ y x τ y y ) = ( x ∂ u ∂ x 0 0 − t ∂ v ∂ y ) {\displaystyle {\begin{pmatrix}\tau _{xx}&\tau _{xy}\\\tau _{yx}&\tau _{yy}\end{pmatrix}}={\begin{pmatrix}x{\frac {\partial u}{\partial x}}&0\\0&-t{\frac {\partial v}{\partial y}}\end{pmatrix}}} represents
861-619: Is the force applied and A is the cross-sectional area. The area involved corresponds to the material face parallel to the applied force vector, i.e., with surface normal vector perpendicular to the force. Wall shear stress expresses the retarding force (per unit area) from a wall in the layers of a fluid flowing next to the wall. It is defined as: τ w := μ ∂ u ∂ y | y = 0 , {\displaystyle \tau _{w}:=\mu \left.{\frac {\partial u}{\partial y}}\right|_{y=0},} where μ
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#1732787825059902-767: The Alps , in the Republic of Ireland ( County Leitrim , County Monaghan , County Mayo and County Cavan ), in Northern Ireland ( County Fermanagh , County Armagh , and in particular County Down ), Germany, Hindsholm in Denmark, Finland and Greenland . The majority of drumlins observed in North America were formed during the Wisconsin glaciation . The largest drumlin fields in the world formed beneath
943-480: The Last Glacial Period . Recently formed drumlins often incorporate a thin "A" soil horizon (often referred to as "topsoil" which accumulated after formation) and a thin "Bw" horizon (commonly referred to as " subsoil "). The "C" horizon, which shows little evidence of being affected by soil forming processes (weathering), is close to the surface, and may be at the surface on an eroded drumlin. Below
984-802: The Laurentide Ice Sheet and are found in Canada — Nunavut, the Northwest Territories, northern Saskatchewan, northern Manitoba, northern Ontario and northern Quebec. Drumlins occur in every Canadian province and territory. Clusters of thousands of drumlins are found in: In the United States , drumlins are common in: Drumlins are found at Tiksi , Sakha Republic , Russia. Extensive drumlin fields are found in Patagonia . A major drumlin field extends on both sides of
1025-595: The Strait of Magellan covering the surroundings of Punta Arenas' Carlos Ibáñez del Campo Airport , Isabel Island and an area south of Gente Grande Bay in Tierra del Fuego Island . Land areas around Beagle Channel host also drumlin fields; for example Gable Island and northern Navarino Island . In 2007, drumlins were observed to be forming beneath the ice of a West Antarctic ice stream. Shear stress Shear stress (often denoted by τ , Greek : tau )
1066-559: The strain rate in the fluid, where the viscosity is the constant of proportionality. For non-Newtonian fluids , the viscosity is not constant. The shear stress is imparted onto the boundary as a result of this loss of velocity. For a Newtonian fluid, the shear stress at a surface element parallel to a flat plate at the point y is given by τ ( y ) = μ ∂ u ∂ y , {\displaystyle \tau (y)=\mu {\frac {\partial u}{\partial y}},} where Specifically,
1107-765: The C horizon the drumlin consists of multiple beds of till deposited by lodgment and bed deformation. On drumlins with longer exposure (e.g. in the Lake Ontario drumlin field in New York State) soil development is more advanced, for example with the formation of clay-enriched "Bt" horizons. Besides the Icelandic drumlins mentioned above, the literature also documents extensive drumlin fields in England, Scotland and Wales, Switzerland, Poland, Estonia ( Vooremaa ), Latvia , Sweden, around Lake Constance north of
1148-430: The addition of soft sediment to a core. Thus, accretion and erosion of soft sediment by processes of subglacial deformation do not present unifying theories for all drumlins—some are composed of residual bedrock. There are two main theories of drumlin formation. The first, constructional , suggests that they form as sediment is deposited from subglacial waterways laden with till including gravel, clay, silt, and sand. As
1189-435: The drumlin forms, the scrape and flow of the glacier continues around it and the material deposited accumulates, the clasts align themselves with direction of flow. It is because of this process that geologists are able to determine how the drumlin formed using till fabric analysis, the study of the orientation and dip of particles within a till matrix. By examining the till particles and plotting their orientation and dip on
1230-411: The erosive action of horseshoe vortices around obstacles in a turbulent boundary layer. Semi-submerged or drowned drumlins can be observed where rising sea-levels flooded the low-lying areas in between drumlin ridges. The most notable example of this is Clew Bay in the west of Ireland , which contains hundreds of drumlin islands and islets. It was once a field of drumlins that was "drowned" following
1271-430: The flat plate above mentioned), states that shear tensor (a second-order tensor) is proportional to the flow velocity gradient (the velocity is a vector, so its gradient is a second-order tensor): τ ( u ) = μ ∇ u . {\displaystyle {\boldsymbol {\tau }}(\mathbf {u} )=\mu {\boldsymbol {\nabla }}\mathbf {u} .} The constant of proportionality
List of tumps - Misplaced Pages Continue
1312-413: The gradient of the velocity profile at the wall, then multiplying by the dynamic viscosity would yield the shear stress. Such a sensor was demonstrated by A. A. Naqwi and W. C. Reynolds. The interference pattern generated by sending a beam of light through two parallel slits forms a network of linearly diverging fringes that seem to originate from the plane of the two slits (see double-slit experiment ). As
1353-591: The ice. Drumlins may comprise layers of clay , silt , sand, gravel and boulders in various proportions; perhaps indicating that material was repeatedly added to a core, which may be of rock or glacial till . Alternatively, drumlins may be residual, with the landforms resulting from erosion of material between the landforms. The dilatancy of glacial till was invoked as a major factor in drumlin formation. In other cases, drumlin fields include drumlins made up entirely of hard bedrock (e.g. granite or well- lithified limestone ). These drumlins cannot be explained by
1394-405: The liquid phase from microelectrodes under limiting diffusion current conditions. A potential difference between an anode of a broad surface (usually located far from the measuring area) and the small working electrode acting as a cathode leads to a fast redox reaction. The ion disappearance occurs only on the microprobe active surface, causing the development of the diffusion boundary layer, in which
1435-445: The margin of glacial systems, and within zones of fast flow deep within ice sheets , and are commonly found with other major glacially-formed features (including tunnel valleys , eskers , scours, and exposed bedrock erosion ). Drumlins are often encountered in drumlin fields of similarly shaped, sized and oriented hills. Many Pleistocene drumlin fields are observed to occur in a fan-like distribution. The long axis of each drumlin
1476-424: The other main theory of formation could be true. The second theory proposes that drumlins form by erosion of material from an unconsolidated bed. Erosion under a glacier in the immediate vicinity of a drumlin can be on the order of a meter's depth of sediment per year, depending heavily on the shear stress acting on the ground below the glacier from the weight of the glacier itself, with the eroded sediment forming
1517-658: The shear stress. Thus, the maximum shear stress will occur either in the web of maximum shear flow or minimum thickness. Constructions in soil can also fail due to shear; e.g. , the weight of an earth-filled dam or dike may cause the subsoil to collapse, like a small landslide . The maximum shear stress created in a solid round bar subject to impact is given by the equation τ = 2 U G V , {\displaystyle \tau =2{\sqrt {\frac {UG}{V}}},} where Furthermore, U = U rotating + U applied , where Any real fluids ( liquids and gases included) moving along
1558-450: The viscosity tensor ( μ x x μ x y μ y x μ y y ) = ( x 0 0 − t ) , {\displaystyle {\begin{pmatrix}\mu _{xx}&\mu _{xy}\\\mu _{yx}&\mu _{yy}\end{pmatrix}}={\begin{pmatrix}x&0\\0&-t\end{pmatrix}},} which
1599-466: The viscosity was ( μ x x μ x y μ y x μ y y ) = ( 1 u 0 0 1 u ) {\displaystyle {\begin{pmatrix}\mu _{xx}&\mu _{xy}\\\mu _{yx}&\mu _{yy}\end{pmatrix}}={\begin{pmatrix}{\frac {1}{u}}&0\\0&{\frac {1}{u}}\end{pmatrix}}}
1640-423: The wall shear stress is defined as τ w := τ ( y = 0 ) = μ ∂ u ∂ y | y = 0 . {\displaystyle \tau _{\mathrm {w} }:=\tau (y=0)=\mu \left.{\frac {\partial u}{\partial y}}\right|_{y=0}~.} Newton's constitutive law , for any general geometry (including
1681-544: The word: Drumlins Drumlins occur in various shapes and sizes, including symmetrical (about the long axis), spindle, parabolic forms, and transverse asymmetrical forms. Generally, they are elongated, oval-shaped hills, with a long axis parallel to the orientation of ice flow and with an up-ice (stoss) face that is generally steeper than the down-ice (lee) face. Drumlins are typically between 250 and 1,000 m (820 and 3,280 ft) long and between 120 and 300 m (390 and 980 ft) wide. Drumlins generally have